1. Field of the Invention
The present invention relates to an electric power steering system which controls the driving of an electric motor on the basis of the steering state of a motor vehicle represented by a steering torque, a steering angle, a yaw rate and the like.
2. Description of the Related Art
In an electric power steering system, the rotation of a steering wheel is transmitted to a steering mechanism via a steering shaft. A steering assist force is transmitted to the steering mechanism from an electric motor in accordance with a steering angle, a steering torque, a yaw rate and the like.
There has been proposed an electric power steering system in which a housing accommodating a steering torque sensor, a steering torque sensor signal processing circuit (sensor module) and an electric motor driving circuit (motor module) is mounted integrally on an electric motor so that a wiring distance is reduced for easy assembling thereof (see Japanese Unexamined Patent Publication No. 8-11730 (1996)).
In this electric power steering system, the sensor module and the motor module are mounted together with a single microprocessor on the same board. Therefore, the entire board should be redesigned every time the specifications of the steering torque sensor or the electric motor (e.g., the capacity of the electric motor for a vehicle model) are changed. Hence, there is a need for designing a board for each vehicle model, requiring a greater effort for the designing. This leads to cost increase.
It is therefore an object of the present invention to provide an electric power steering system which includes a sensor module and a motor module physically separately provided therein, so that the sensor module and the motor module can separately be produced, and a sensor such as a steering torque sensor, a steering angle sensor or a yaw rate sensor and an electric motor can independently be adjusted for correction of variations thereof.